2008_tugun bypass tunnel_13th atc 2008

7/30/2019 2008_Tugun Bypass Tunnel_13th ATC 2008

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Tugun Bypass Tunnel Using Top-Down Cut-and-Cover Method

J Hsi1, S Lambert

2and M Thomas

3

ABSTRACT

The Tugun Bypass Tunnel in Gold Coast, Australia was constructed usingdiaphragm walls with the top-down cut-and-cover method to allowsimultaneous construction of an airport runway extension above thetunnel, whilst excavation of the tunnel continued underneath. The tunnelwas built in an environment of high groundwater table and deep depositsof alluvial and estuarine soils with the toes of the walls founded in soildeposits. There was a potential risk for differential settlements to occurbetween the diaphragm wall panels, caused by the runway fill placed overthe tunnel roof during excavation. Dewatering within the diaphragm wallswas required to facilitate the construction of the tunnel. The tunnel wasalso built in an area where environmental considerations were of greatimportance. Three-dimensional numerical modelling was undertaken topredict the differential settlements of the tunnel with considerationsof varying subsurface profile, staged excavation and dewatering, non-uniform surface loading and complex soil-structure interaction. Fieldinstrumentation and monitoring was implemented to confirm numerical

predictions.

INTRODUCTION

The Tugun Bypass is a new four-lane motorway ofapproximately 7 km in length, connecting south-east Queenslandand north-east New South Wales, Australia. One of the mainfeatures of the project is a tunnel of about 334 m in length,constructed near one end of the Gold Coast Airport runway.

The tunnel was constructed using the top-down cut-and-coverconstruction method with diaphragm walls installed to supporttemporary excavation and form permanent walls of the tunnel.As the tunnel was constructed near one end of the runway, therewas a height restriction on the construction plant and equipment.Twin low headroom cutters and a hydraulic grab were selectedfor the construction of the diaphragm walls. These walls wereconstructed in alluvial and estuarine deposits comprising sandsand clays with the groundwater table close to the surface.

During excavation of the tunnel, the airport runway was to beextended above the tunnel, which involved filling over the tunnelroof. The loads acting on the roof were carried by the diaphragmwalls, which were located entirely within the alluvial andestuarine deposits. Some settlements would occur, resulting fromthe additional loading of the runway extension and loss of roofslab bearing and wall skin friction due to excavation. To maintainthe wall and roof structural integrity, differential settlements inthe longitudinal and transverse direction of the tunnel needed tobe minimised.

This paper also discusses other key features of the tunnel,including environmental management in sensitive areas,dewatering and recharging of the groundwater during excavation,diaphragm wall construction methods, waterproofing of thetunnel, challenges of working under the obstacle limitationsurface (OLS), and cathodic protection of the reinforcement.

PROJECT OVERVIEW

The 7 km long Tugun Bypass is a four-lane motorwayconnecting Currumbin in Queensland to Tweed Heads in New

South Wales, Australia. The highway deviates off the existing

Pacific Motorway at Stewart Road, traverses hilly terrainthrough Tugun Hills and floodplains adjacent to the Gold CoastAirport, and merges with Pacific Highway at Kennedy Drive.The key feature of the project is the tunnel of 334 m in length(Ch5588 to Ch5922.4), with approach ramps to the north andsouth of the tunnel. The project also involves cuttings of up to30 m in depth, five bridges, four soil nail walls and a largequantity of earthworks. The bypass caters for potentialexpansion to a six lane motorway subject to future trafficvolume. Figure 1 shows the project route plan.

The Tugun Bypass project was awarded to the PacificLinkAlliance (PLA) in January 2006 under an alliance style contractfollowing a competitive tendering process. The alliance team,consisting of Department of Main Roads Queensland, AbigroupContractors and SMEC Australia, was responsible for the designand construction of the project. A suballiance, consisting ofPiling Contractors Bauer Joint Venture (PCBJV), was engaged to

construct the tunnel diaphragm walls. The estimated value of theproject was approximately A$500 million. The project met thedeadline of November 2006 for the surface handover to the GoldCoast Airport Authority for construction of the runwayextension. The remaining construction works continued,targeting the scheduled contract completion date of December2008. Following commissioning of the road, there is a ten yearmaintenance period as part of the alliance contract.

GEOLOGY AND GEOTECHNICALCHARACTERISTICS

Geology

The Tugun Bypass alignment is characterised by two geologicalsettings. In the northern section, the alignment traverses hillyterrain consisting of Neranleigh Fernvale Beds of the Beenleigh

13th Australian Tunnelling Conference Melbourne, VIC, 4 - 7 May 2008 1

1. Chief Technical Principal, SMEC Australia Pty Ltd, Level 6, 76Berry Street, North Sydney NSW 2060. Email: [email protected]

2. Project Manager, Abigroup Ltd, 924 Pacific Highway, Gordon NSW2072. Email: [email protected]

3. Senior Tunnel Engineer, SMEC Australia Pty Ltd, Level 2, 60

Leichhardt Street, Spring Hill Qld 4000.Email [email protected]

CH100

CH7200

CH5922.4CH5922.4

CH5588

CH100

CH7200

FIG 1 - Project route plan.


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block comprising lithic sandstone (referred to as greywacke),slate, metasiltstone (argillite) and chert. The rocks developed inthese zones have been subjected to varying degrees ofdeformation and typically identified by coherent strata beingdiscontinued by a tectonic mlange. Highly sheared material upto 10 m wide, which contains varying size blocks of countryrock (eg chert, greenstone, siltstone and sandstone) bounding themlange zone, are typical. Groundwater is not usually

encountered in the northern section.The tunnel is situated in the southern flood plain, which is

subject to periodical flooding. The geology in the southernsection comprises Neranleigh Fernvale Beds overlain byCenozoic estuarine and coastal deposits. These deposits are up to35 m in thickness, comprising river gravels, sand and clay andflood plain and tidal delta muds and silts. At the tunnel location,the subsurface horizons consist of dune sands, Coffee Rock(local term given to cemented silty sands), estuarine interbeddedclays and sands and residual soils derived from the weatheredbedrock. Groundwater is slightly saline due to the closeproximity to the ocean. The water table is influenced by bothtidal movements and rainfall events recharging CobakiBroadwater. Due to low-lying ground surfaces ranging fromRL 0.5 m to RL 4.0 m (AHD), potential exists for acid sulfate

soils.

Subsurface profile

As the subsurface conditions varied spatially along the lengthand width of the tunnel, extensive site investigations usingboreholes (BH) and piezocones (CPTU) were undertaken at thewall and barrette locations. Within the footprint of the runwayextension, the investigations were done at a spacing ofapproximately 20 m intervals. The plan of the site investigationis shown in Figure 2.

Based on the probe hole information, the subsurface wasdivided into discrete soil units, classified according to materialtype and consistency or density. There was Coffee Rock (CR)found in the tunnel areas. Coffee Rock is a local term used to

describe a layer of cemented silty sand having dark browncoffee-like colour. Contrary to its name, this material has soilcharacteristics, and its relative densities are typically mediumdense or better, although loose Coffee Rocks were alsoencountered occasionally. Geotechnical stratigraphy at the tunnelsite is summarised as follows (top down):

Topsoil thin skinned (


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ISSUES AND CONSTRAINTS

Construction of a tunnel in soft ground at shallow depths isconventionally undertaken using the cut-and-cover method.However, to allow for construction of the runway extension thatoccurred concurrently with the tunnel excavation, the top-downconstruction method was employed. Diaphragm walls and castin situ tunnel roof slabs were chosen to facilitate the construction

requirements and time constraints. Figure 2 shows the footprintof the runway extension oblique to the tunnel alignment.

Following the handover of ground surface, up to 2 to 3 m offill for the airport runway extension was placed above the tunnelroof. Loads acting over the entire width of the roof slabs weretransferred directly to the diaphragm walls and the barrettes. Thesite investigations revealed presence of estuarine depositsconsisting of loose materials below the toe of the walls.Therefore, there was a potential for the tunnel to settle duringexcavation. One of the critical issues was the differentialsettlements between the walls and the central barrettes, and alongthe walls. These differential settlements could potentially inducesignificant stresses in the roof structures and in the walls. Otherissues in relation to the tunnel construction are listed below:

Obstacle limitation surface (OLS) applied at both ends ofthe runway to provide safe airspace for approaching aircraft.

This required all construction activities to be undertakenwithin a headroom of as low as 8 m. Use of cranes orheavy-lifting equipment was only allowed outside the airportoperating hours.

High groundwater level due to its close proximity to the seaand Cobaki Broadwater. The groundwater was practically atthe ground surface level. A reliable dewatering system wasessential during excavation.

Environmental requirements strict environmental controlswere enforced such that drawdown of the groundwater tableoutside the diaphragm walls was minimised. All acidicsulfate soils excavated from the tunnel had to be dried andneutralised with lime prior to placement as fill inembankments. The existing ecological conditions alsoneeded to be enhanced.

CONSTRUCTION METHODS

Suitable construction methods were chosen to address the issuesand constraints mentioned above. In order to adhere to the OLSrequirements, special low headroom hydraulic grab (LeibherrHS852HD) and 2.8 m wide trench cutter (CBC25) were used.The guide walls were built first followed by construction of the6 m wide primary panels (steps one to five of Figure 4) and 2.8 m


TUGUN BYPASS TUNNEL USING TOP-DOWN CUT-AND-COVER METHOD

FIG 3 - Subsurface profile.

Average RLat top (m)

Thicknessrange (m)

Materialtype

Consistency/density

sat

(kN/m3)'

('

()k

(m/day)E50

refandEoed

ref(MPa)Eur

ref

(MPa)

0.5 3.3 - 5.5 Sand Very loose 18 30 0 1.0 10 30

-4.0 4.8 - 11.0 CR Mediumdense

19 32 2 0.1 50 150

-11.2 1.5 - 3.0 CR Dense 20 34 4 0.1 80 240

-13.5 1.2 - 5.8 Sand Loose 18 32 2 1.0 30 90

-17.5 0.0 - 6.5 Clay Stiff 18 28 0 1 10- 4 10 30

-21.1 0.5 - 3.3 Sand Loose 18 32 2 1.0 30 90

-23.0 3.2 - 7.2 Clay Firm 18 24 0 1 10- 4 7 21

-28.6 1.3 - 3.0 Clay Very stiff 19 29 0 1 10- 4 25 75

-30.8 - Bedrock - - - - - - -

Note: sat is saturated unit weight; ' is drained friction angle; ' is drained dilatation angle; k is saturated permeability; E50ref

is secant Youngs modulus ata reference pressure of 100 kPa; Eoed

refis tangent Youngs modulus for primary odometer loading at a reference pressure of 100 kPa; E ur

refis

unloading/reloading Youngs modulus at a reference pressure of 100 kPa; cohesion c= 0 kPa for all soil types; Poissons ratio = 0.3 and ur = 0.2(unloading/reloading) for all soil types; power m used in hardening soil model for stress level dependency is 1.0 for clay and 0.5 for Coffee Rockand sand.

TABLE 1

Geotechnical parameters.


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wide secondary panels (steps six to eight of Figure 4). The opentrench was supported by bentonite slurry, when the cutterundertook full excavation (steps two to three and six). A steelreinforcement cage was lowered when the panel was excavatedto full depth (steps four and seven). Concreting of the panels wasthen achieved by the tremie method (steps five and eight).Figure 4 presents the construction sequence of the diaphragmwall.

Following completion of the diaphragm walls and barrettes,dewatering and excavation commenced inside the walls.Excavation was initially undertaken to depths of up to about RL-2 m to allow construction of the roof slab. A watertightmembrane was installed as part of the waterproofing system.When the roof slab was completed, it was backfilled and the sitewas cleared for handover to the Gold Coast Airport. Theseactivities commenced in April 2006 after environmentalapprovals were granted and were completed by November 2006,which was the scheduled date of the site surface handover.Excavation below the runway extension and construction of thetunnel continued thereafter.

STRUCTURAL DETAILS

Tunnel box

The tunnel structure consisted of diaphragm walls forming theouter walls and barrettes along the centre line of the tunnel. Thediaphragm walls were 1 m in thickness and extended from theNorthern Portal (Ch5588) to the Southern Portal (Ch5922.4). Thewalls were installed to the depth of RL -17 m from the top of theguide wall at RL 2 m. The internal width between the diaphragmwalls ranged from about 25.7 m at the northern portal to 28 m atthe southern portal. Barrettes were 0.8 m thick and 2.8 m wide(longitudinal) with a clear spacing of 2.8 m throughout thecentral axis of the tunnel, extending to RL -17 m in depth.

These structures had a 100 year design life, using N-gradereinforcing steels and 50 MPa high strength concrete. There wereno mechanical joints at the interface of the primary and

secondary panels in the longitudinal direction. The roof and base

slabs were cast in situ structures connected to the diaphragmwalls by reinforcement couplers in a typical moment connection.The clear height was 6.1 m with minimum vehicular headroomof 5.3 m within the carriageway envelope. A series of threeniches, where the roof slab was slightly elevated, wereconstructed in the roof for mechanical jet fan provision. Nichedimensions were in plan view 23 m long across the full width ofthe tunnel and were evenly spaced along the length of the tunnel.The thickness of the overburden including the runway pavement

was up to 4 m and decreased to 1 m at the tunnel portals.The base slab was 1 m thick with a founding level ranging

from RL -5.5 m to RL -9.5 m. The base slab had all drainageprovision cast monolithically within the slab to mitigate potentialfor pipe breakages caused by differential movement and buoyancyforces. Figure 5 shows the typical cross-section of the tunnel.Cathodic protection provision, sheet and joint waterproofing,mechanical/electrical service provision and monitoringinstrumentation were also cast into the tunnel structure.

Approach ramps

The north and south approach ramps were both constructed bythe bottom-up construction method. The south ramp was 281 mlong and 24 m wide at the portal tapering out to 44 m at the topof the ramp to accommodate a slip lane. The north ramp was271 m long and 28 m wide.

Diaphragm walls were used to form part of the permanentramp structures within 32 m from the portal, followed bytemporary sheetpiles to allow construction of the ramps in thenarrow corridor. The extended diaphragm walls were 27 m deepand made up one wall of the deep sumps, which were located ateach portal. Sump dimensions were 31 m long (longitudinal) and16 m wide (transverse) with an average depth of 5.5 m below thebase slab and were cast in situ elements. One side of the sumpwas coupler connected to the extended diaphragm walls.

The walls and ramp slabs were cast in situ elements withprovision for cathodic protection, waterproofing, monolithicdrainage, mechanical/electrical services and tension screwpiles

in the sump and ramp slabs.

4 Melbourne, VIC, 4 - 7 May 2008 13th Australian Tunnelling Conference

J HSI, S LAMBERT and M THOMAS

FIG 4 - Diaphragm wall construction sequence (courtesy of Piling Contractors Bauer Joint Venture).


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Screw piles

Tension piles were required to prevent flotation of the approachramps. The piles were designed for loads from the one in 100year flood and a design life of 100 years. Maximum upwardmovement under long-term load was limited to 25 mm to ensurethat the interface stresses between the ramp and tunnel portal didnot exceed allowable values.

Steel screw piles were selected due to the tight site constraints,the least noise impact on neighbouring properties and thesignificant cost advantages resulting from being a very efficientpile in resisting tension. Piling could be carried out day and nightwith simple equipment (excavator rig and hydraulic motor),which provided flexibility to the construction program.

Site trials were performed to determine the most appropriateparameters for the pile. The typical configuration adopted was ashaft of diameter 219 mm and thickness 8.2 mm, fitted with ahelix of a diameter of 600 mm and a thickness of 32 mm. Inweaker ground the helix was increased to 700 mm diameter andpreboring was required in areas where dense or very dense sandwere encountered in order to overcome the installation limits.For longer piles where preboring was not possible the shaftthickness was increased to 12.7 mm for increased installationtorque capacity.

A sacrificial thickness of 4 mm was incorporated to allow forcorrosion over the 100 year design life. The working load was inthe range of 300 to 350 kN and typical pile lengths were 9.0 to13.5 m, with rows of eight to ten piles placed at spacings varyingfrom 2.4 to 4.0 m. A typical cross section of screw pilearrangement is shown in Figure 6. Installation torques were usedto control pile founding depths based on results from preliminarytest piles where a correlation was derived between torque andSPT values in various ground conditions.

Corrosion monitoring elements were installed at 12 locationsthroughout each ramp, set within 0.5 to 1 m of the piles andat varying depths of up to 7 m. At three locations additionalsacrificial piles were installed each side of the ramp with adetailed, accurate recording of pile section thickness. These pilescould later be extracted for measurement as part of the overallcorrosion monitoring system. Bonding bars connected thereinforcement at the top of each pile (which was isolated fromthe ramp slab reinforcement) to allow for future use of cathodicprotection if required.

Waterproofing

The approach ramps and tunnel were made watertight by a fullybonded membrane system applied to the permanent structure



Screw Pile

Base Slab

FIG 6 - Screw piles under approach ramp.

Roof Slab

Diaphragm Wall

Base Slab

Barrette

Waterproof Membrane

Waterproof Membrane

FIG 5 - Typical tunnel cross-section.


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externally (see Figure 5). The diaphragm walls did not receive anexternal membrane, but rather a watertight joint between theprimary and secondary panels was achieved through the jointtreatment (ie tongue and groove) from the CBC25 low headroomdiaphragm wall cutter, as shown in Figure 7.

The 1.5 mm thick membrane Bituthene 3000, manufactured byGrace Construction Products, was cold applied in two layers.The self-adhesive membrane, with 100 micron cross-laminatedHDPE film and rubber bitumen compound, was applied to75 mm thick blinding concrete beneath slabs, or directly toexternal face of approach ramp walls. The membrane in the wallapplication was protected by protection board placed prior to thebackfilling operations.

All joints within the structure received a hydrophilic waterstop,with a volumetric increase of greater than 200 per cent when incontact with water. In addition, an injection tube system capableof reuse and through which both grout and resin injection couldtake place was located adjacent to the waterstop.

Cathodic protection

The tunnel diaphragm walls and barrettes were protected againstcorrosion by an impressed current cathodic protection (ICCP)system using anodes connected to a dc power source. The systemadopted a total of 58 vertical anode wells extending to 30 mdepth. The Wenner Method of measuring soil resistivities wasadopted to gather the necessary design parameters of depth,thickness and resistivity of soil layers. The positive dc output

terminal was connected via cables to the anode array while thenegative output was connected to the diaphragm wall andbarrette reinforcing steel bars. For typical arrangement of theICCP system refer to Figure 8.

A corrosion monitoring system was implemented for theapproach ramps and tunnel slabs utilising a Moncor CorrosionMonitoring unit. The system was capable of providing informationto assess the level of corrosion of steel reinforcement and earlydetection of chloride contamination. The monitoring units wereimbedded within the concrete at representative locations in thestructure and took measurements of the relative degree of chlorideingress into the concrete, the corrosion rate and the corrosionactivity potential of the steel reinforcement. The levels of relativechloride ingress and corrosion rate were measured at a junctionbox using a specialised external instrument.

A monitoring system was also installed to the approach rampscrewed piles, with provision made for the future connection ofthe screw piles to an ICCP system. The monitoring systemlocated a corrosion probe, zinc reference, silver/silver chloridereference and titanium reference electrode at depth beneath thepermanent structure. Electrical continuity was maintained via thesteel reinforcement within the concrete structures with a negativeconnection welded to the steel screw piles. The probes,electrodes and negative connections terminated at junction boxesfor monitoring using special equipment for corrosion ratemonitoring and reference electrodes potential.

DEWATERING SYSTEM

The dewatering system for the tunnel works consisted of

dewatering wells, a chemical treatment system, settling pondsand recharge wells. The system was required to allowconstruction of the tunnel, which at its deepest was more than10 m below sea level and at its closest point was only 60 m fromthe Cobaki estuary system.



primary panel primary panel

secondary panel

cutter

H - beam to be removed

during concreting

FIG 7 - Joints between diaphragm wall panels (courtesy of Piling

Contractors Bauer Joint Venture).

FIG 8 - Cathodic protection arrangement.


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The dewatering system was installed in stages to suit thecomplex nature of the tunnel construction. The initial dewateringwells were installed to permit excavation for roof construction.The dewatering pipe work for the main tunnel excavation wasinstalled under the roof slab such that in the next stage of worksthe dewatering pipe work would be suspended from the roof slab.This enabled dewatering of the main tunnel zone ahead of theexcavation works and allowed the excavation works to progress

without disruption to the dewatering system. A total of21 dewatering wells were installed with the deepest wellsinstalled to RL -25 m, approximately 10 m below the deepesttunnel excavation level.

The extraction wells consisted of an outer screened casing,coarse sand filter and an inner extraction pipe. Air pumps at thebase of each well lifted the groundwater to the surface where itwas piped to one of the two treatment plants at either end of thetunnel. Electric and diesel compressors at each of the treatmentplants provided the compressed air to run the pumps. Air pumpswere chosen because of their simplicity. They had no movingparts and as such were very reliable and essentially maintenancefree.

The extracted water was high in iron (40 to 60 mg/L) and ifleft untreated would quickly clog the recharge wells with iron

precipitates. The pH and iron levels were also unsuitable fordirect surface discharge, so that treatment was essential. In thetreatment plant the pH of the groundwater was raised to 8.0 to9.0 by injection of a caustic soda (NaOH) solution. At theelevated pH the iron began to precipitate out of the solution. Thereaction was relatively fast, so after a short contact time the pHwas brought back down to 6.5 to 7.5 using hydrochloric acid(HCl). This halted the process of iron precipitation and alsoreacted with the caustic soda to form common salt (NaCl). Atthis stage the iron had come out of the solution and was asuspended solid.

The processed groundwater was then pumped to a series ofsettlement ponds where the iron floc was allowed to settle out.Additional treatment could be conducted in the ponds to improveturbidity or to adjust the pH to meet the discharge criteria. The

treated water was then pumped to the recharge system.The recharge system consisted of a storage tank, used to limit

and maintain a constant head to the recharge wells, and a bank ofrecharge wells connected by a system of pipe work, both air andwater. There were two separate recharge systems for each end ofthe tunnel. The treated water was gravity fed from the headertank to the recharge wells. These consisted of a screened casingsurrounded by a gravel pack installed to a depth of around 20 m.The top of the well used unscreened casing and a bentonite plugto allow the well to be pressurised. The pressure was limited to1 - 2 m of head as the natural ground consisted mainly ofremnant dune sands and under high pressures the sand tended toliquefy and could blow out at the surface. Recharge flows weregenerally in the range of 1 - 1.5 L/s per well. Excess water fromthe recharge system was discharged as surface water atdesignated discharge points.

Although most of the iron was removed in the treatmentprocess and in the settling ponds the recharge wells needed to beback flushed at regular intervals to remove iron build-up on thescreens and in the gravel packs. This was done using compressedair, which was fed to the bottom of the well by an internal airline. The iron-rich water, which was back flushed to the surface,was then returned to the dewatering system. The air used to backflush the wells was supplied from the same compressors thatpowered the air pumps for the dewatering system.

As the tunnel works progressed and sections became fullysealed the dewatering wells were decommissioned by cutting thedischarge pipe at the top of the tunnel base slab, the air pumpwas extracted, the conduit sealed with an expandable bladder and

the hole grouted back to slab level.

GEOTECHNICAL ANALYSIS

Design considerations

Geotechnical design of the tunnel was required to address thefollowing three key issues:

Excavation support during construction the diaphragm wallstructures were designed to ensure stability of the excavation.

Issues included structural design of the walls, base heave,hydraulic uplift, piping and liquefaction.

Structural integrity due to settlement of the tunnel duringconstruction the tunnel was subjected to loading fromairport runway fill, which resulted in settlements. Theinfluences of differential settlements on structural capacitywere assessed.

Long-term stability and serviceability of the tunnel buoyancy of the tunnel when the groundwater table was closeto the surface or the flood level.

Two-dimensional numerical modelling

Design of the tunnel was initially undertaken using the finite

element software PLAXIS (Version 8.4) at selected sections.This numerical package was used to analyse two-dimensionalplane-strain conditions involving complex soil-structureinteraction for the design of the structural members. Structuralbeam elements were used to simulate the diaphragm walls.Global factor of safety during each of the construction stages wascalculated based on the c- reduction method to ensure theminimum FoS was achieved. The software allowed modelling ofconstruction sequence, changing groundwater levels and varyingsubsurface conditions across the width of the tunnel.

Three-dimensional numerical modelling

A three-dimensional numerical modelling package, PLAXIS 3DFoundation (Version 1.6), was employed to predict the

settlements of the tunnel caused by runway fill loading andexcavation. Due to the limitation of the program, settlementanalyses were undertaken in sections, each of approximately 40 -60 m in length. The major advantages of the three- dimensionalmodelling were as follows:

Ability to model the physical dimensions of the wall andbarrette structures. This improved the accuracy of settlementprediction as it accounted for longitudinal stiffness ofthe tunnel, which assisted in load redistribution and toeresistance of the structures.

Ability to simulate three-dimensional load distribution wherethe runway fill was placed oblique to the longitudinal axis ofthe tunnel.

Ability to model three-dimensional subsurface profile based

on probe holes at discrete locations.

Ability to simulate dewatering within the tunnel excavationarea.

The hardening soil (HS) model was considered mostappropriate to simulate soil behaviour in an excavation. The HSmodel took into account unloading and reloading behaviour andirreversible plastic strains of soil. The HS stiffness parameterswere defined with respect to a reference pressure of 100 kPa. Thekey parameters included E50

ref, Eoedref and Eur

ref as shown inTable 1. The published data indicated the ratio of Eoed

ref to E50ref

is about 0.7 to 1.4 and the ratio of E urref to E50

ref varied from twoto four. The analysis adopted E50

ref= Eoedrefand Eur

ref= 3E50ref.

Presented here is a 41.2 m long section of the tunnel betweenCh5728.8 and Ch5770. This section of the tunnel was at the

deepest location of the tunnel, beneath the thickest layer of the




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runway fill and underlain by sloping bedrock level and changingclay thickness. A jet fan niche of approximately 12 m long(longitudinal) also lay within the centre of this section, whichwas also incorporated in the three-dimensional model. Withinthis chainage range, there were seven boreholes. Due to thecapacity of the program, four representative boreholes, whichwere evenly distributed spatially, were selected for the analysis.The assumed subsurface profiles are shown in Table 2.

Assumptions of analysis

The construction sequence was considered in the analysis tosimulate the load transfer from the runway fill to the diaphragmwalls and barrettes. The assumed construction sequence is

described below:1. application of loads exerted on the virgin ground from the

working platform built to RL 2 m (for construction of theguide walls) and construction load of 10 kPa;

2. installation of diaphragm walls and barrettes to RL -17 m;

3. removal of the working platform and application of 10 kPaconstruction load on the ground surface;

4. dewatering and excavation to underside of the roof slab;

5. installation of the roof slab (and jet fan niche), and backfillto existing ground surface;

6. placement of runway fill to design heights (simulated aspressures) with 10 kPa live load above the runway;

7. staged dewatering and excavation within the diaphragmwalls to underside of the base slab;

8. casting of the base slab and completion of the tunnelstructure; and

9. return of the groundwater table to the ground surface andremoval of 10 kPa surface loads.

The settlement-induced impact was assessed for the abovestage seven, which was considered most critical with maximumexcavation under full runway loading.

The assumed levels within the modelled chainage range aresummarised in Table 3.

Results of analysis

The deformed mesh of the three-dimensional finite element

analysis under the full runway loading and at the final stage ofthe excavation is shown in Figure 9. The predicted settlementprofiles (Class A prediction) at the top of the roof slab along thediaphragm walls and barrettes prior to casting of the base slabare presented in Figure 10.

The predicted settlement (Class A) of the tunnel duringexcavation was about 45 mm on the left-hand side (LHS), 43 mmon the right-hand side (RHS) and 35 mm along the centralbarrettes. The maximum differential settlement was predicted tobe 12 mm between the walls and the barrettes. To allow foruncertainties, the tunnel was designed for a maximumdifferential settlement of 25 mm. The structural analysis showedthat the longitudinal in-plane stiffness of the tunnel wouldsmooth out differential settlements along the tunnel alignment,with the presence of the jet fan niche and variability of thesubsurface conditions.

Field performance

The performance of tunnel during construction was assessedbased on the field monitoring results. This was a means toconfirm that the structural integrity of the diaphragm walls andbarrettes were not adversely affected by differential settlements.



Chainage range 5728.8 to 5737.6 5737.6 to 5743.6 5743.6 to 5755.2 5755.2 to 5761.2 5761.2 to 5770.0

Feature RL (m)

Natural ground level 0.5 0.5 0.5 0.5 0.5

Top of roof slab -0.8 -0.25 +0.4 -0.25 -0.8

Bottom of roof slab -1.8 -1.25 -0.6 -1.25 -1.8

Initial excavation -2.8 -2.8 -2.8 -2.8 -2.8

Initial dewatering -3.8 -3.8 -3.8 -3.8 -3.8

Intermediate excavation -6.0 -6.0 -6.0 -6.0 -6.0

Intermediate dewatering -7.0 -7.0 -7.0 -7.0 -7.0

Top of base slab -8.4 -8.4 -8.4 -8.4 -8.4

Bottom of base slab -9.4 -9.4 -9.4 -9.4 -9.4

Final excavation -9.7 -9.7 -9.7 -9.7 -9.7

Final dewatering -11.7 -11.7 -11.7 -11.7 -11.7

Toe of diaphragm wall -17.0 -17.0 -17.0 -17.0 -17.0

TABLE 3

Assumed geometry during construction.

Borehole #1 #2 #3 #4 Soil type(density/

consistency)Location LHS RHS LHS Centre

Chainage 5730 5737 5757 5768

Unit RL at top of each layer

1 0.5 0.5 0.5 0.5 Sand (VL)

2 -4.8 -3.4 -5.0 -2.8 CR (MD)

3 -10.8 -14.4 -9.8 -9.7 CR (D)

4 -13.8 -15.9 -12.6 -11.7 Sand (L)

5 -17.8 -17.1 -17.5 -17.5 Clay (St)

6 -24.3 -21.8 -20.8 -17.5 Sand (L)

7 -27.3 -25.1 -21.8 -18.0 Clay (F)

8 -33.3 -30.8 -25.0 -25.2 Clay (VSt)

9 -36.3 -32.0 -27.4 -27.4 Bedrock

Note: LHS is left-hand side of tunnel facing increasing chainage direction;RHS is right-hand side of tunnel; centre is centre line of tunnel.

TABLE 2

Subsurface profiles.


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Three instrumentation arrays were set up at Ch5655, Ch5718 andCh5770, corresponding to locations of the runway fill (seeFigure 11).

Each array consisted of three settlement plates placed abovethe LHS and RHS diaphragm walls and the central barrettes (seeFigure 12). These were installed prior to runway fill placementand excavation of the tunnel in order to capture all construction-induced movements. In addition to the settlement plates, survey

targets were also installed at inner walls to the tunnel to recordtunnel movement during excavation. This information wascalibrated against the settlement plate measurements as the initialtunnel movement record was not available.

Figure 13 shows a summary of construction activities,recorded settlements and the predicted settlements at diaphragmwall and barrette locations at Ch5718. The settlement predictionadopted here is the result of analysis between Ch5728.8 andCh5770. Monitoring commenced at the beginning of November2006. Excavation of the tunnel commenced in mid December



FIG 9 - Deformed three-dimensional finite element mesh.

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Distance along Centre Line (m)

Settlement(mm)

Central Barrettes

RHS Diaphragm Wall

LHS Diaphragm Wall

CH5728.8 CH5770

FIG 10 - Predicted settlement profiles at top of roof (Class A prediction).

FIG 11 - Plan of instrumentation arrays.


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2006 from the Northern Portal at Ch5588. The excavationprocess reached Ch5718 in early January. Placement of runwayfill above CH5718 followed in mid January, which had resultedin visible settlements of the tunnel. The settlements appeared to

have ceased after the excavation reached final depth in midFebruary. The monitoring data showed that the field performanceof the tunnel was consistent with the predictions obtained fromthe PLAXIS 3D foundation modelling. Maximum differentialsettlements between the barrettes and the diaphragm walls wereless than 25 mm at all stages of construction.

ENVIRONMENTAL MANAGEMENT

The Tugun Bypass was regulated by a number of complexCommonwealth, state and local government environmental laws.The environmental impact assessment required prior to approvalinvolved extensive environmental surveys and subsequent routeoption analysis. Surveys included assessment of meteorological,hydrological, geological and biological aspects of the

environment. Ecologically sustainable development was achievedby avoiding, minimising, mitigating, and/or compensating forenvironmental impacts, in that order of precedence.

Three former landfill sites had to be remediated beforeconstruction commenced. As the sites straddled the boundary ofthe road corridor, remedial works were undertaken inconjunction with the Gold Coast Airport. On-site containment ofthe waste was deemed impractical and waste removal involvedexcavation and transportation of 11 190 tonnes of material to a

licensed disposal facility. The remedial works minimised thepotential migration of groundwater pollution from the area andgroundwater monitoring was undertaken quarterly to confirmthat ongoing contaminant migration did not occur.

A total of 12 500 m2 of concrete paving was dug up, crushedand reused on site within culverts as an anti-erosion measure andas general fill throughout the site. Approximately 200 m3 ofconstruction waste was taken off site every month and sent to a

recycling depot where 80 per cent of it was recycled. Up to onemillion litres of water a day was required during hot summermonths for dust control on site. Where possible this water wassourced from sediment basins where site run-off was collected.All mulched vegetation was reused on site for the landscapingand revegetation and also for the stabilisation of exposed topsoiland substrate.

Vegetation clearing along the alignment was undertaken in acontrolled and methodical manner, with very few breaches of thedelineated clearing limit. A combination of well signpostedclearing limits and ongoing education of staff ensured that allwere aware of the environmental significance of the surroundingvegetation and fauna habitat. Environmental managementmeasures were implemented prior to any clearing works andthese included the provision of a professional fauna handler todeal with any fauna found during the clearing.

Treatment of the groundwater associated with the tunnel worksis described above in the Dewatering System section.

CONCLUSIONS

The Tugun Bypass tunnel was constructed under many strictconstraints, including the obstacle limitation surface (OLS), thickalluvial and estuarine soil deposits, groundwater table at shallowdepths, sensitive environment, early handover of site surface forrunway extension, etc. Diaphragm walls with the top-downconstruction techniques were adopted for the construction of thetunnel structure. Detailed geotechnical investigations andnumerical modelling were undertaken for design optimisation andrisk minimisation. A dewatering and recharge system wasimplemented to lower the groundwater table within the excavation

area and minimise the water table drawdown outside the tunnel.Waterproofing and cathodic protection of the tunnel structure wereundertaken to meet the long-term serviceability and durabilityrequirements. Screw piles were used under the approach ramps toresist buoyancy from the groundwater. Strict environmentalmanagement was employed to manage waste material andcontaminants and maintain the ecological environment.Instrumentation and monitoring during tunnel construction haddemonstrated satisfactory performance of the tunnel.



FIG 12 - Typical instrumentation section.

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0

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26/11/2006 16/12/2006 5/01/2007 25/01/2007 14/02/2007 6/03/2007 26/03/2007 15/04/2007

Date

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0

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4

Measured at LHS

Measured at Centre

Masured at RHS

RL of Excavation

RL of Top of Fill

Predicted 35 mm (Centre)

Predicted 43 mm (RHS)

Predicted 45 mm (LHS)

Se

ttlemen

t(m

m)

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fE

xcava

tionan

dTopo

fF

ill(m

)

FIG 13 - Settlement monitoring results at Ch5718.

2008_tugun bypass tunnel_13th atc 2008

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